1. Technical Field
The present invention relates to a position detection system for optically detecting the position of a designating member, and a display system with input function including the position detection system.
2. Related Art
As a position detection system for optically detecting the position of a designating member, for example, a system is proposed in which plural light sources are provided at positions away from each other and when detection light is emitted from each of the plural point light sources toward the designating member via a light-transmissive member, the detection light reflected by the designating member is transmitted through the light-transmissive member and is detected by a light receiving element (see JP-T-2003-534554). Another position detection system is proposed in which detection light emitted from each of plural point light sources is emitted via a light guide plate and the detection light reflected by the designating member is detected by a light receiving element (see JP-A-2010-127671 and JP-A-2009-295318).
In such position detection systems, a common light receiving element is used and the position of designating member is detected based on the result of a comparison between a received light intensity at the light receiving element when one or some of the plural point light sources turn on and a received light intensity at the light receiving element when another or some others of the plural point light sources turn on.
However, with the configurations disclosed in JP-T-2003-534554, JP-A-2010-127671 and JP-A-2009-295318, it is difficult to expand the detection range even when two-dimensional coordinate data (for example, X coordinate data and Y coordinate data) of the designating member is to be obtained. That is, if the detection range is broadened, light becomes incident on the light receiving element from a direction that is greatly inclined from the direction of a normal line to a light receiving surface. However, since the light-receiving element is directional in such a way that the sensitivity to oblique directions is low, it is difficult to expand the detection range with the configurations disclosed in JP-T-2003-534554, JP-A-2010-127671 and JP-A-2009-295318. Here, the present inventor is studying a technique of emitting detection light L2 from a light source unit 12 radially along an imaginary plane P12 parallel to a reference surface 41a and then detecting, by a light receiving element 13, reflected light L3 reflected by a designating member Ob, thus detecting the position of the designating member Ob within the reference surface 41a as an angular position about the light source unit 12, as shown in FIG. 15. The inventor is also studying a technique of providing a retroreflection portion Sb on an outer peripheral surface of the designating member Ob and also arranging the light receiving element 13 in such a way that the direction of a normal line L130 to a light receiving surface 130 becomes parallel to the reference surface 41a and the imaginary plane P12, thus causing the reflected light L3 from the designating member Ob to become incident efficiently on the light receiving surface 130.
However, in the configuration shown in FIG. 15, an intensity L of the detection light L2 reaching a position at a distance d from the light source unit 12 is inversely proportional to the square of the distance d, as expressed by the following equation.L=L0/d2 where L0=exit intensity from the light source unit 12
Therefore, when the distance d between the designating member Ob and the light source unit 12 is short, for example, de, the output level from the light receiving element 13 is sufficiently high. However, when the distance d between the designating member Ob and the light source unit 12 is long, for example, df, the output level from the light receiving element 13 is low and detection accuracy is lowered. Meanwhile, if the exit intensity from the light source unit 12 is increased, when the distance d between the designating member Ob and the light source unit 12 is de, which is short, the output level from the light receiving element 13 is too high and exceeds an allowable input range of a detection circuit.